1. Field of the Invention
This invention relates generally to a signal level control circuit and more particularly to such a circuit in which a resistive layer insulator semiconductor field effect transistor is employed as a variable impedance element.
2. Description of the Prior Art
A resistive layer insulator semiconductor field effect transistor, as the name implies, is a field effect transistor having a resistive layer overlying an insulating layer on a semiconductor substrate. This field effect transistor has an extremely low distortion characteristic as compared with conventional field effect transistors, CdS photo-conductive impedance elements, etc. when used as an impedance element.
Therefore, it has been proposed to use the resistive layer insulator semiconductor field effect transistor as a variable impedance element in a signal attenuation circuit as disclosed in a co-pending application Ser. No. 358,168, filed May 7, 1973. However, in such an attenuator circuit, the gain of the circuit does not become a linear function of the variable D.C. control voltage applied to the gate electrodes of the resistive layer insulator semiconductor field effect transistor.
Further, in such an attenuator circuit, the response speed of the circuit to the control voltage cannot be designed fast by reason of a condenser circuit connected to the resistive layer insulator semiconductor field effect transistor.